Basic DC electricity question

[HK]

I'm trying to help my daughter with a school project and while I know my way around house wiring, I'm a relative newbie when it comes to low voltage.

Let's say I have a 6 volt DC power supply (4 "C" batteries). If I want to drive a small motor and some lights, I just find 6 volt motors and lights and wire them in parallel or series.

Why is it that I can't light up a 12v light with a 6 volt power supply? I always test my batteries using a multi-meter and as the battery ages, the voltage drops. With low voltage, the device (flashlight, etc.) still works but the light is weak. So, wouldn't a 12v light just be weak if I use a 6v power supply?

Is there some kind of voltage threshold at which a device won't work?

Like I said, basic question.

[w_tom]

Light bulbs work at may voltages. For example, a 120 volt incandescant bulb will work at 60 volts AND last for maybe 100 years continuous. As voltage drops, bulb life expectancy increases exponentially (about a factor of 12). As voltage drops, bulb intensity also decreases exponentially. Sure, the 12 volt bulb will work at 6 volts. But its light output will be massively diminished and its efficiency is decreases (less light per amp of electricity).

So yes, a 12 volt lamp will work > I'm trying to help my daughter with a school project and while I know my way

[Toller]

It is an interesting question actually. A 15w 12v bulb, when wired to a 6v battery draws .6a. Apparently your batteries can't produce .6a, so the bulb doesn't light up. Wouldn't be very bright anyhow.

>

[Bob]

You're on the right track. The bulb filament glows from the amount of current through it. Too much current and the bulb is overbright or blows. Not enough current and the bulb glows weak or not at all. Current is a function of voltage divided by resistance. Higher voltage bulbs have higher resistance filaments. Therefore 6v divided by higher resistance equals less current and less glow.

Bob

[dpb]

And, to follow up, OP could take the bulb and his VOM and measure the resistance across the filament and see what it is to see what the current draw should be...or maybe the bulb isn't good, even? :)

Or, of course, take enough batteries in series to make 12V ...

[Doug Miller]

Yes.

No.

The filament resistance is too high, and/or your 6 volt source doesn't supply enough current. Are you trying to light up an automobile light bulb with four AA batteries?

It depends on the current as well as the voltage.

Depends on the device. For something as simple as a light bulb, I wouldn't think so. Try an experiment: see if you can light up an ordinary 60W incandescent light bulb (120V) from a car battery (12V).

[Doug Miller]

Since power increases in proportion to voltage, but increases in proportion to current *squared*, don't you suppose he's better off to put the batteries in parallel to increase the current?

[Jack]

In parallel you are dividing the current. In series you are dividing the voltage.

There is no threshold for the incandescent lamp, after all its an analog device and light output is varied from zero to 100% continuous with respect to varying the voltage. You don't see it because you haven't enough current to heat the filament up - see if you could see it in the dark. I'm sure if you have a 6V car or motorcycle battery instead of the C cells, the 12v lamp would light up.

[HK]

Thanks everyone. Good information here.

This is a Grade 6 project and I was a little distressed to find that even the basic concepts of electricity haven't been taught yet they are supposed to build a basic circuit and working model.

For instance, my daughter assumed that to power two 6v lights, she needed a

12v power supply. Yikes.

Is Grade 6 too early to teach Ohm's Law? Should they be learning to hook up basic circuits without learning Ohm's Law and the concepts of voltage, current and resistance?

Some of this stuff is coming back to me. Am I correct in assuming that the problem with wiring the lights in series is there is too much cumulative resistance at the far end of the circuit to light the lights?

FYI, we're building a small airboat. There are four 3v LED lights powered by 2 "C" batteries and a single 6v motor powered by 4 "C" batteries.

Any suggestions on a fun followup project that will help both of us learn more electricity concepts?

[dpb]

...

But, since by the same token, I=V/R and R is what he's given w/ the particular bulb, P ~ V^2/R so same can be said of voltage...

I was only commenting wrt to the fact OP has a 12V-rated bulb and is/ was 'spearmint-ing so was only one of many alternatives to try and observe what happens, certainly not the only one..."Best" in any sense wasn't at all a part of the suggestion.

[Chop Suey]

HK inspired greatness with:

You do know that LED's must be wired in series with a resistor to make them glow, right?

[Bob]

Nothing wrong with that as long as they are in series.

No. a good mix of math and science.

Go back to Ohms law. 2X resistance = 1/2 current

Aahhh... Remember that we've been talking incandescent light bulbs. A LED is a diode and not a bulb even though it gives off light. Although the same principles apply since LED's use a dropping resistor to limit current, similar to items in parallel.

Google or your local library.

[Bob]

OOPS. I meant series. Now I'm getting tongue tied.

[Chris Friesen]

I agree that is the proper way to do it.

However, they work just fine by themselves if the voltage supply is low enough, or if you parallel enough LEDs together.

Chris

[Goedjn]

[Goedjn]

Actually, they don't. They have to be wired in series with a resistor to keep them from eating your battery whole, but as long as you have enough voltage, they'll light.

You can make the worlds smallest flashlight by just taping the leads of an LED to either side of a watch battery.

[Doug Miller]

So what's the problem? Connect them in series.

Too much cumulative resistance at some point, anyway -- not necessarily all the way at the end.

Browse around at the web site for Edmund Scientific //

of good stuff there.

[Mark Lloyd]

It should light, but dimly and more yellowish.

Try measuring the 6V supply WITH that light connected. You may find that it's loading your C cells so much it's getting a lot less than

6V.

[Mark Lloyd]

The filament won't be as hot, which leads to a change in color (it won't be as white).

Maybe that 12V bulb is a car headlight, which is severely overloading the power supply.

[Mark Lloyd]

That won't increase the current capability. It may still be insufficient for that bulb.

What is the current capacity for new C cells?